Many kinds of photosensitive imaging systems have been developed and found commercial application. The most common is photographic film, manufactured using a thin emulsion containing silver halide crystals coated onto a plastic or paper support material. Here, photoexposure creates microscopic changes in the silver halide crystal structure. These then serve as nucleation points for the transformation of silver halides into metallic silver during subsequent chemical processing, which develops this “latent image” into an image with far greater contrast and visibility.
Likewise, for integrated device fabrication, photosensitive polymers called photoresists are in common use. These materials are designed to change molecular weight with photoexposure, either by photo-induced crosslinking or photoinduced scission. For the processing of integrated devices, a substrate (often a silicon wafer, and often comprising partially fabricated devices as well) is coated with this polymer, and photoexposure to define a pattern for processing occurs. The regions of photoexposure can be defined using lenses and a photomask or reticle, but can also be done by contact printing with a mask placed directly against the photoresist. The “latent” image after exposure in this case is the polymer film with a variation in density, or molecular weight. The polymers of different molecular weight have different solubilities, and so development occurs when the photoresist is treated with a suitable solvent, and the more soluble material dissolves, leaving the less soluble material behind.
Although many other photosensitive imaging systems have been created, a common feature of all of them is that they place the photosensitive material on the substrate where it is ultimately to be exposed, developed and used.
This can lead to problems, especially for integrated circuit manufacturing. Integrated circuits are fabricated layer by layer, with the circuit elements in each layer defined by a separate lithographic process. After several layers have been manufactured, the circuit is no longer flat, but can have considerable topography. Although common photoresist coating techniques, such as spin coating, can work extremely well for flat, planar surfaces, it can be much more difficult to produce a uniform layer of photoresist on a surface with topography.
There are several reasons that a uniform photoresist layer is desired for lithography. First, it is commonly known that reflections from the front and rear surface of the photoresist layer optically interfere, and the overall reflectivity of the film can range from nearly 0% to almost 90%, depending on the exposure wavelength, coherence, and the layer thickness. Films of different thickness in areas of different topography will have different reflectivity, which means that different exposure doses are required to achieve identical results in the different areas. Furthermore, for the formation of microstructures, the depth of focus is often as small as the typical resist thickness. Focus conditions will therefore be different in areas of different topography.
In addition to this, residues of previous processing steps can contaminate subsequent photolithographic coatings. Chemically amplified resists, for example, are extremely sensitive to amines. A trace amount of amines on the surface can change the lithographic sensitivity by an order of magnitude. Other lithographic applications, such as the formation of amino acid sequences using lithographically defined patterns, may be even more sensitive to this contamination problem.
There have been several innovations to correct this in photolithography. One approach to the problem is to introduce space filling “dummy” features to the design layout, which have no electrical function but serve to make the layer more uniform in profile. This has been used with some success, as described, for example, in “New Data Processing of Dummy Pattern Generation Adaptive for CMP Process” by Shinichi Ueki et al., Proc. SPIE 3748, pp 265–272 (1999). These features are, however, fabricated at the same time as the electrically active features, and must therefore be of the same material as the layer itself (e.g. polysilicon or aluminum). These dummy features are therefore not always electrically inert, but can add unwanted capacitance and inductance to the neighboring circuit.
Other approaches have attempted to increase the depth of focus of the imaging system, using for example interference effects from phase-shifting masks. This has been described in Chapter 5 and the references therein of the book Resolution Enhancement Techniques in Optical Lithography, by Alfred K. K. Wong. This has also proven to be very useful in some circumstances. However, phase shifting masks are not inexpensive, and their use also introduces other problems in the IC design process.
Recently, there has been a new investigation into the limits of contact printing lithography, adapting techniques for stamping mass manufactured compact audio and video disks for stamping microdevices. Some of these have been described, for example, in “Step and Flash Imprint Lithography: A New Approach to High-Resolution Patterning,” by C. G. Willson et al., Proc. SPIE 3676, pp 379–389, (1999) and “Imprint Lithography with 25-Nanometer Resolution” by Stephen Chou et al., Science 272, pp 85–87 (April 1996). These techniques have even been applied to novel curved surfaces in “Patterning curved surfaces: Template generation by ion beam proximity lithography and relief transfer by step and flash imprint lithography,” by C. G. Willson et al, in J. Vac. Sci. Technol. B 17(6), pp 2965–2969, (1999).
Although these reflect a great degree of innovation and creativity, all have in common that a master pattern, such as a photomask or an imprint master, is created, and the pattern then directly transferred by some process into the final material